Radio frequency identification (RFID) devices are becoming increasingly popular for a variety of industrial, retail, transportation, and other applications. RFID technology provides a positive identity for any object, person, or the like, bearing an RFID transponder through the use of a passive, radio frequency signal. In a typical application, an RFID transponder comprises an antenna and an integrated circuit. When a separate RFID reading device broadcasts a radio frequency signal, the signal interacts with the RFID transponder antenna. The transponder antenna converts part of the received RF signal energy into an electrical current. This electrical current powers the integrated circuit. The integrated circuit then modulates its impedance to create a return RF signal. This return RF signal is then detected by the antenna in the RFID reading device. This modulated, RF return signal carries encoded data about the transponder based on the data previously stored in the integrated circuit. For example, the serial number of the transponder may be returned to the RFID reading device via this modulated RF signal. Finally, the RFID reading device decodes the signal returned from the transponder to complete the identification.
RFID transponders are being integrated into a growing number of applications. Employee identity badges, animal identity devices, retail pricing and inventory devices, retail security devices, manufacturing product and material tracking devices, vehicle identification devices, and the like, are just a few examples of the expanding area of applications for RFID technology. RFID transponders are ideally suited for integration with a wide variety of products and in a wide variety of situations. RFID transponders may be purely passive devices where all of the energy for operating the integrated circuit is derived from the broadcast RF signal. Alternatively, active RFID systems may incorporate an on-board battery to provide power to the identity chip and/or power for the transponder's return RF signal. In fixed systems, such as motor vehicle transponders used for automated toll collection, the additional cost of the on-board battery is easily justified by the improved performance of the device. Conversely, in cost sensitive applications such as retail pricing and security tags, the RFID transponder device must be as inexpensive as possible and is therefore, typically, a passive device.
The on-board antenna is a key enabling technology for RFID transponder devices. The broadcast RF energy may be in the form of a magnetic field, an electric field, or a mixed field as in typical radio signal broadcast. The transponder antenna is designed with a shape and a size based on the characteristics of the broadcast RF energy such as the field type and the signal frequency. Moreover, the design of RFID tags typically requires matching the antenna impedance and load impedance, usually by a matching circuit, for maximizing the RF power from the reader's interrogation or command signal received at the tag antenna to be delivered to the RFIC with minimum loss, and thereby achieve optimum tag sensitivity. Theoretically, maximum power delivery is achieved by conjugate impedance matching, which demands that the impedance from the antenna be, as closely as possible, the mathematical conjugate of the RFID input impedance. This represents an ideal impedance match.
In many applications, it is desirable to reduce the overall size or “footprint” of a particular RFID device. The reduced size may be required for inclusion on or in retail goods having small dimensions. Alternatively, it may simply be desirable to make the RFID device as inconspicuous as possible. While technology exists to drastically reduce the size of an IC component of an RFID device, similar miniaturization of the antenna of an RFID device can result in a significant reduction in performance. As stated above, a particular IC and antenna of an RFID device ideally have matched impedance characteristics. By reducing the overall size of the RFID device, and thus the antenna, it may prove difficult to adequately provide the impedance characteristics for efficient function of the device. As such, the RFID may suffer from inefficient power transfer to the IC, a reduced operating range with respect to an interrogator, and a weak return signal in response.
In addition, an antenna connected to a RFID tag is generally designed for operation on a specific or narrow range of substrates to which it may be attached. Other substrates may cause the radiation efficiency of the antenna to deteriorate from the designed optimal mounting substrates. Thus, the antenna, and consequently the RFID device, will no longer function as intended. This loss of antenna efficiency may be due to a number of variable packaging factors. For example, each substrate has its own dielectric and conductive characteristics that typically affect the impedance matching between the wireless communication device and its antenna. Impedance matching ensures the most efficient energy transfer between an antenna and the wireless communication device, as discussed above, and placement of an RFID device in proximity to a surface having dielectric and conductive properties outside of a particular range may reduce the performance of the RFID device. These adverse effects to the performance of an RFID device may also be experienced upon the inclusion or integration of an electronic article surveillance (“EAS”) tag or device. Such EAS devices often include a magneto-acoustic mechanism having one or more metallic components that may subsequently interfere with or reduce the performance characteristics of a particular RFID device.
In view of the above, it would be desirable to provide an RFID device having a reduced footprint while providing for efficient operation on a variety of surfaces and/or in combination with an EAS tag.